This invention generally relates to a process for a gas removal zone, such as an acid gas removal zone. More specifically, this invention relates to improvements in efficiency of such processes in which the level of carbon monoxide is reduced from the carbon dioxide that is being removed.
In gasification applications in which the final product is liquid fuels or chemicals, typically both a fully shifted and a partially shifted or unshifted feed must be treated in a process that removes the sulfur compounds (including H2S and COS) and CO2. Among the processes that can provide such treatment are the Selexol® process—using a mixture of dimethyl ethers of polyethylene glycol (UOP LLC, Des Plaines, Ill.), the Rectisol® process—using a methanol solvent (licensed by both Linde A G, Polach, Germany and Lurgi A G, Frankfurt Am Main Germany), the Sulfinol® process—using a mixture of sulfolane and an aqueous solution of either di-isopropanol amine or methyl-diethanol amine (Jacobs, Pasadena, Calif.), the Flexsorb® process—using a proprietary solvent (ExxonMobil Research and Engineering, Fairfax, Va.), the Morphysorb® process—using a mixture of n-formylmorpholine and n-acetylmorpholine (Uhde GmbH, Dortmund, Germany) and the Purisol® process using N-Methyl-2-Pyrrolidone (NMP) (Lurgi A G, Frankfurt Am Main Germany). Each of these processes employs a solvent that absorbs the sulfur compounds and/or carbon dioxide from an acid gas.
The most straightforward set-up for these types of processes is a separate train for both feeds. From a capital cost stand-point it is advantageous to have separate H2S and CO2 absorbers for the 2 feeds and common equipment for the remainder of the process. These set-ups are typically able to meet the sulfur specs for the treated gases and product CO2 without problems. However, the electricity requirements for a CO2 recycle compressor within the process can become excessive as the CO specification in the product CO2 is reduced below 1 mol-%. In current applications 1000 ppmv CO specifications for the product CO2 are becoming the industry norm. The difficulty in keeping CO out of the product CO2 is due to the high levels in the partially shifted or unshifted feed and the relatively large absorption of CO in the H2S and CO2 absorbers for this feed. The large quantities of recycle gas from the CO2 recycle compressor ultimately increase the semi-lean and lean solvent requirements and associated utilities such as refrigeration and reboiler duty to undesirable levels as well. An additional restriction on the treated fully shifted syngas that makes some options for limiting the CO in the product CO2 unusable due to a limit on the CO contamination that is allowable from the partially shifted or unshifted gas. Other designs were disclosed in U.S. application Ser. No. 12/566,822 filed Sep. 25, 2009 in which the 1000 ppmv CO specification for the product CO2 is maintained while minimizing utility requirements by transferring the absorbed CO from the partially treated or untreated syngas to the fully shifted syngas. However, these designs are sometimes unacceptable because despite reductions in CO in the CO2 stream, they can increase the CO in the treated fully shifted syngas by 20 to 30%.